The present invention relates generally to power supplies, and specifically to efficient switch mode AC/DC power supplies with high power factor and low AC ripple at its output.
For efficient operation and low harmonic content, AC/DC switch mode power supplies commonly use an active power factor corrector (PFC) to make the AC input current track the input voltage waveform shape. (When the power factor is one, the voltage and current are exactly in phase, with the same shape, so that the ratio between the voltage and the current is that of a pure resistance.) Typically, active PFC is implemented by switching the AC input current at high frequency, using pulse width modulation (PWM) so that the current waveform approximates the AC voltage waveform as closely as possible. The switch output is smoothed by an L/C filter to give a DC voltage, but there is invariably residual AC ripple at line frequency harmonics. It can be shown that using a single switching stage with a power factor approaching one and a finite output capacitance, it is impossible to eliminate the AC ripple completely.
For this reason, switch mode power supplies with PFC frequently use a two-stage architecture, in which the first stage is designed to achieve a high power factor, while the second stage removes the residual AC ripple. (The dual-stage design is also useful in bringing the output voltage to any desired level and improving the dynamic behavior of the power supply.). The first stage may comprise, for example, a boost-type converter with DC first-stage output set to 400 V and ripple in the range of 5-30 V peak-to-peak. The second stage comprises a DCxe2x80x94DC converter, with its own PWM controller and switch, which receives and down-converts the DC output voltage of the first stage to the desired output supply voltage. The second stage uses a feedback loop from the supply output to the PWM controller in order to attenuate the ripple.
This two-stage topology is costly to implement and suffers from low efficiency. Each independent stage must switch the full voltage and current of the supply. In other words, there is double conversion of the full supply power. Therefore, the total power losses of a two-stage supply are roughly double those of a single-stage switch-mode supply with comparable output power and per-stage efficiency. The total of the major cost components (including magnetic elements, large capacitors, power diodes, heat sink, printed circuit board area, etc.) of the two-stage supply are, likewise, almost double those of the single-stage equivalent.
It is an object of some aspects of the present invention to provide efficient switch-mode power supplies with high power factor and low ripple.
In preferred embodiments of the present invention, a DC power supply comprises a switch-mode power stage, which converts an AC input voltage to a DC output voltage, together with an output regulator for sensing and removing the ripple from the DC output voltage. Preferably, the power stage is designed and controlled to achieve high power factor, as is known in the art. The regulator receives as its input not the entire DC output voltage, as in two-stage supplies known in the art, but rather only a small fraction of the DC output voltage, equal roughly to the amplitude of the peak-to-peak ripple generated by the power stage. Based on this input, the regulator generates a correction waveform of proper amplitude and phase to cancel the ripple in the DC output voltage.
Preferably, for high efficiency, the regulator comprises a switch, which operates by pulse width modulation (PWM) based on feedback from the power supply output. Although the regulator switches the full current of the power supply, it must typically switch only the small fraction of the output voltage that it receives. It therefore dissipates far less power than the second (ripple attenuating) stage of a two-stage power supply, as described above. As a result, the novel power supply of the present invention is able to achieve high PFC and low ripple, comparable to two-stage supplies known in the art, with far higher efficiency.
An output regulator as described herein may be used in conjunction with substantially any type of power stage known in the art, implementing any suitable method for PFC. Even when high power factor is not a key design objective, output regulators in accordance with the present invention may still be used for efficient ripple cancellation, such as in a power supply having a small output capacitance. Different types of output regulators may be used, as well, including linear regulator designs when very low ripple is desired and efficiency is not the paramount concern. The PWM operation of the regulator can be controlled using standard PWM controllers, digital controllers or fuzzy-logic type controllers.
There is therefore provided, in accordance with a preferred embodiment of the present invention, power supply apparatus, including:
a power factor corrector (PFC) unit, which is adapted to receive an AC voltage from an AC source, and to smooth the AC voltage while adjusting a waveform of an AC input current from the AC source relative to the AC voltage so as to generate an output PFC voltage made up of a DC component with a residual AC ripple while the power factor measured at the unit input is close to one (approaching the theoretical maximum); and
a regulator, coupled to receive an indication of a ripple amplitude and a ripple phase of the residual AC ripple and to generate, responsive thereto, a correction voltage which is combined with the PFC voltage to generate a DC output voltage in which the AC ripple is substantially reduced relative to the PFC output ripple voltage.
Preferably, the PFC unit includes a switch, which is coupled so that when the switch is closed, the AC input current flows through the switch, and a control circuit, which is coupled to open and close the switch so as to adjust the waveform of the AC input current. Most preferably, the control circuit is adapted to control the switch so as to apply a pulse-width modulation (PWM) to the AC input current with a duty cycle selected so as to cause a desired adjustment of the input current waveform. Alternatively, the control circuit is adapted to control the switch so as to apply constant-on-time control or constant duty cycle/variable frequency control to the AC input current.
Preferably, the control circuit is coupled to receive a feedback input indicative of the PFC output voltage, and to open and close the switch responsive to the feedback, input. Additionally or alternatively, the control circuit is coupled to receive a control input indicative of at least one of the AC voltage and the AC input current, and to open and close the switch responsive to the control input.
In a preferred embodiment, the PFC unit includes a transformer including primary and secondary windings, which are coupled so that the AC input current flows through the primary winding, while the PFC voltage appears across the secondary winding.
Preferably, the regulator includes a power input circuit, which is coupled to provide a regulator input current, a switch, which is coupled to the power input so that when the switch is closed, the regulator input current flows through the switch, and a control circuit, which is coupled to receive the indication of the ripple amplitude and the ripple phase, and to open and close the switch responsive to the indication in order to generate the correction voltage. Further preferably, the PFC unit includes a first inductor through which the AC input current flows, and the power input circuit includes a second inductor, which is magnetically coupled to the first inductor so as to generate the regulator input current. Most preferably, the power input circuit further includes a rectifier, which is coupled to the second inductor so as to rectify the input current for input thereof to the switch.
Alternatively, the power input circuit is coupled to receive the AC voltage from the AC source in parallel with the PFC unit. Preferably, the regulator includes a transformer including primary and secondary windings, which are coupled so that the regulator input current flows through the primary winding, and the correction voltage appears across the secondary winding.
Preferably, the regulator includes one or more reactive circuit elements, which are coupled together with the switch in a buck-type regulator configuration or, alternatively, in a boost-type regulator configuration. In a preferred embodiment, the regulator is coupled in series with the PFC unit so that the power input circuit receives the PFC voltage, and the control circuit is operative to open and close the switch with a duty cycle selected so that the output voltage exceeds the PFC voltage by a difference voltage that is approximately equal to a peak-to-peak value of the ripple amplitude.
In an alternative embodiment, the regulator includes a linear regulator.
Preferably, the correction voltage has a correction amplitude that is substantially equal to the ripple amplitude and a correction phase that is substantially opposite to the ripple phase, and the correction voltage is added to the PFC voltage in order to generate the output voltage. Most preferably, the PFC unit includes a PFC output capacitor having first and second terminals, and wherein the regulator includes a regulator output capacitor having a third and fourth terminals, wherein the third terminal is connected to the second terminal, and the PFC unit is configured to output the PFC voltage across the PFC output capacitor, while the regulator is configured to output the correction voltage across the regulator output capacitor, so that the DC output voltage is provided between the first and the fourth terminals.
Preferably, the correction voltage includes one of a positive voltage and a negative voltage or, alternatively, both a positive and a negative voltage.
In a preferred embodiment, the regulator is further adapted to process the DC output voltage so as to generate an AC correction input to the PFC unit.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for supplying DC power, including:
smoothing an AC voltage received from an AC source while performing power factor correction on a waveform of an AC input current from the AC source relative to the AC voltage, so as to generate a PFC voltage made up of a DC component with a residual AC ripple;
receiving an indication of a ripple amplitude and a ripple phase of the residual AC ripple;
generating, responsive to the indication, a correction voltage; and
combining the correction voltage with the PFC voltage to generate a DC output voltage in which the AC ripple is substantially reduced relative to the PFC voltage.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: